Answers

Will all applications developed on Windows 8 be runnable on Windows XP/Vista/7 and vice versa?

Or are there any exceptions?

Sorry if it's a stupid question, I just want to make sure before I download it :)

Not a stupid question at all, and something that is important to most Windows users. The answer is that nearly all applications that run on Windows 7 will continue to work on Windows 8. There are always a few exceptions. Some categories of
applications, such as Security, Anti-Virus, and low-level system utilities generally need to be updated for every new Windows release. They tend to have hooks into OS internal data structures that are not "exported" for applications to rely upon, and have
to change as Windows evolves. Microsoft works closely with these partner companies to ensure that products can be ready to meet their business goals and customer expectations.

If you do find any problems with applications that you depend on, please share that information here and our Compatibility team will take note.

Yes, legacy applications will run fine on windows 8 however, I cannot say that it is true the other way around. I would assume that, no, windows 8 apps will NOT run on any other windows version because they are coded in HTML 5 and Javascript which
runs natively in windows 8.

Will all applications developed on Windows 8 be runnable on Windows XP/Vista/7 and vice versa?

Or are there any exceptions?

Sorry if it's a stupid question, I just want to make sure before I download it :)

Not a stupid question at all, and something that is important to most Windows users. The answer is that nearly all applications that run on Windows 7 will continue to work on Windows 8. There are always a few exceptions. Some categories of
applications, such as Security, Anti-Virus, and low-level system utilities generally need to be updated for every new Windows release. They tend to have hooks into OS internal data structures that are not "exported" for applications to rely upon, and have
to change as Windows evolves. Microsoft works closely with these partner companies to ensure that products can be ready to meet their business goals and customer expectations.

If you do find any problems with applications that you depend on, please share that information here and our Compatibility team will take note.

It seems clear that Metro apps developed for Windows 8 will NOT run on earlier versions of Windows.

But no doubt developers will still be able to write apps using traditional languages and APIs to run on the
desktop. With so many remaining XP users it would be silly to ignore them just yet.

The real question is whether people will continue to buy NEW apps built for the LEGACY desktop. Will Photoshop CS6 or CS7 be a Metro app? Even from Microsoft, will new apps ALL be Metro? Will the next Office be Metro? Is it expected
that all future apps are supposed to be Metro?

Please make sure to report these issues to Microsoft. Microsoft invests a lot of resources on Application and Driver Compatibility testing and partner outreach. The community feedback is an important channel to expand this effort.

I completely agree with the fact that there are too many XP users out there. I personally always keep a dual-boot system on every computer I own and at least one of the operating systems on it is XP. I have watched Microsoft fade out previous
versions of windows by making it impossible to get support for them. Here it seems that I am watching yet another OS from Microsoft that seems to be geared to eliminate previous systems. My biggest worry comes with the ability to work with Office
regardless of which OS I use. Maybe I am speaking out of line being as I have not quite come to the conclusion of what "Metro" is and how it determines the difference between the OS's of XP/Vista/7. Is it as large as a jump from FAT32/NTFS or is
it something smaller like a version upgrade from Java? Don't mean to sound on the technogically illiterate side of life, I tend to do quite a bit for software troubleshooting and virus/spyware removal for a lot of people I know and it has become increasingly
difficult to manually delete some virus files with the amount of extra "security" that Microsoft has added to the newer OS as they come out. Any news on how many "windows only" needed files that will be able to hide temp files with viruses in them?

On another note, my father uses a program called PaperPort on a regular basis. I tried to install it on 8 dev preview and it managed to lock up about 2/3 of the way through the install. I am not sure as to how to pull up any errors when an install
just stops and doesn't give an excuse behind it. Any knowledge on that would be awesome.

As a final note, I hope that XP never gets completely fased out because I already have a hard enough time finding drivers to work in XP on newer model systems such as laptops that can't just have different hardware put in them that is supported by XP.
Sometimes even manual installation with .dll's becomes an issue so hopefully enough people can stand to have XP continued as a supported OS simply because it is the first stable system I know of that Microsoft can claim credit for.